Rockets launched from beneath the water generally do not ignite the rocket motor until the rocket has cleared the water surface. During time between launch and ignition the nozzle of the rocket motor can be positioned by a hydraulic actuator system so that rocket orientation can be corrected on ignition.
Ignition of the rocket motor after underwear launch has been shown to produce undesirable stress forces on the rocket motor exit nozzle. These stress forces on the exit nozzle are transferred to the hydraulic actuator system and to the flexseal between the nozzle and the combustion chamber, potentially causing structural failure.